An example of known constant speed control device for a vehicle is shown in FIG. 1. That is, FIG. 1 shows a conventional constant speed control device disclosed in, for example, Japanese Patent Application Laid-open No. 39811/1983, in which 1 depicts a set switch responsive to an operation of a vehicle operator to start a constant speed running, 2 a cancelling switch responsive to an actuation of a braking device (not shown) to terminate the constant speed running and 3 a speed sensor for detecting a running speed of the vehicle, which comprises a rotary member 3a having four magnetic poles and adapted to be rotated by a meter cable (not shown) for transmitting rotation of a vehicle transmission (not shown) and a reed switch 3b and produces a series of pulses having a frequency proportional to a running speed. 4 depicts a vehicle mounted battery, 5 a main switch which is a power source switch for supplying electric power from the battery 4, and 6 a control device having a processing circuit 6a including a microcomputer etc. and is supplied with electric power by closing the main switch 5. The control device is responsive to signals from the set switch 1, the cancelling switch 2 and the speed sensor 3 to perform various operations necessary to automatically equalize an actual running speed Vs to an aimed speed Vr and to output various control signals. 7 depicts a motor-driven throttle actuator responsive to the various control signals from the control device 6, 8 a suction passage of an engine and 9 a throttle valve provided in the suction passage 8 and ganged with an acceleration pedal (not shown). The throttle valve is driven in open and close directions by a link 7a and a wire 7b of the throttle actuator 7. A rotation angle of the link 7a corresponding to the degree of opening of the throttle valve 9 is detected by a potentiometer (not shown) housed therein and given to the control device 6 as a signal indicative of a throttle position. The link 7a and a motor for driving the throttle actuator are connected to each other by an electromagnetic clutch (not shown and disconnected from each other by an electromagnetic clutch signal from the control device 6.
In operation, when the main switch 1 is closed by the vehicle operator and electric power is supplied from the battery 4, the control device 6 commences its operation to process an output of the speed sensor 3 which is a series of pulses having a frequency proportional to the vehicle speed V. The control device 6 measures a pulse period of the output of the speed sensor 3 to obtain an actual speed Vs. The operator operates the set switch 1 to supply the speed signal to the control device 6 which stores the value Vs as the aimed speed Vr and commences a constant speed control.
Thereafter, the control device 6 compares the aimed speed Vr with the actual speed Vs obtained from time to time and provides a control signal with which the throttle actuator 7 is driven to regulate the opening of the throttle valve 9 to thereby make the actual vehicle speed coincident with the aimed speed Vr. That is, when the actual speed Vs is lower than the aimed speed Vr, the control device 6 provides a throttle open signal to drive the throttle valve to a desired position and, when the actual speed Vs is higher than the aimed speed Vr, provides a throttle close signal to close the valve 9 suitably. Thus, it is possible to make the vehicle speed constant without an acceleration pedal operation by the operator. When the operator actuates the brake device while such constant speed control is performed, the cancelling switch 2 is actuated to supply a termination signal to the control device 6 for terminating the constant speed control. Upon the termination signal, the control device 6 provides a disconnection signal for disconnecting the electromagnetic clutch, immediately, upon which the throttle actuator 7 disconnects the electromagnetic clutch. Thereafter, the operator is allowed to control the vehicle speed by regulating the throttle opening by means of the acceleration pedal.
In the conventional constant speed control device constructed as above, when the set switch 1 is operated to make the vehicle speed constant, the speed signal obtained by the control device 6 on the basis of the output of the sensor 3 is not coincident with a value indicated on a speed meter mounted on the vehicle. The reason for this is that the speed signal which is used to perform the constant speed control must be highly responsive to the vehicle speed while the response of the speed meter is intensively lowered since, otherwise, a vibration of indicator needle occurs which is uncomfortable for the operator. Therefore, the vehicle may run at a speed other than that set by the operator according to an indication on the speed meter. Particularly, an aimed speed tends to be much different from that indicated on the speed meter after an automatic acceleration or deceleration is performed by means of switches etc. other than the acceleration pedal during running at a constant speed.